Lateral Variability of Turbidite-Debrite Couplets in Submarine-Lobe Fringes: Example From the Maastrichtian Lewis Shale, Washakie Basin

Abstract

Turbidite-debrite couplets, which create heterogeneity and negatively affect hydrocarbon-reservoir properties, have been documented in cores and outcropped fringes of submarine lobes during last few years. The 3-D variability of these couplets in lobe fringes is, however, poorly known. In this study, the vertical and lateral lithofacies variations in the submarine-fan fringes are examined at bed to submarine-fan scales using 4 cores as well as gamma-ray logs of about 1,500 closely spaced wells in Washakie Basin, Wyoming. Basin-floor submarine-fan lobes, and thereby lobe fringes are identified by the lobate planform on sandstone thickness maps, thickness of structureless sandstones, and upward coarsening gamma-log motif. Vertical lithofacies-transition trends in axis, fringe, and distal parts of submarine lobe complexes are identified by Markov chain analysis for about 1,200 lithofacies units of cores. In distal fringes, mud-clast-bearing muddy sandstones commonly overlie cleaner structureless sandstones. The common transition (without intervening erosional surface) between structureless sandstone (representing turbidite) and mud-clast rich muddy sandstone (i.e., debrite) indicates that sedimentation on distal lobe fringes was typically associated with flow transition from turbidity current to debris flow. In contrast, such flow transition was not significant on the lateral fringe of fans. The relatively frequent transition from sandstone to mud-clast-rich sandstone on lateral fringes implies that high-concentration sandy turbidity currents were maintained there as the dominant type of flow along the lateral fringe. However, fully transitional debris flows, derived from turbidity currents by entrainment of surrounding muds, tend to develop mud-clast-rich muddy sandstones preferentially in the distal fringe of submarine lobes due to the longer runout distance along main flow direction. Therefore, in spite of the general heterogeneity along submarine-fan lobe fringes, the lateral fringes are characteristically sandier than distal fringes. Although the main hydrocarbon flow trends along the amalgamated channel sandstones of the lobe axis on fan reservoirs, the second preferred flow direction depends on the connectedness of sandy lateral fringes and lobe centers from different lobes, i.e., a direction generally transverse to the axis of the submarine fan.

AAPG Datapages/Search and Discovery Article #90291 ©2017 AAPG Annual Convention and Exhibition, Houston, Texas, April 2-5, 2017